Mass spectrometry is a powerful analytical tool used for measuring the molecular mass of analytes in a sample. When using a time-of-flight mass spectrometer, the speed of flight of the ions is about 107 times faster than the speed of migration of molecules in an electrophoretic gel, therefore mass spectrometry offers an extremely fast analytical method, even when measurement of the spectrum is repeated 10 to 100 times to achieve a good signal-to-noise ratio.
Analysis by mass spectrometry typically begins with the samples being ionized by any number of means, for example, by Matrix Assisted Laser Desorption Ionization (MALDI) or Electrospray Ionization (ES). MALDI preparation and measurement procedures consist first of embedding the analyte molecules on a sample carrier in a solid, IR or UV-absorbent matrix which is usually an organic acid. The sample carrier comprising the matrix and analyte is placed in the ion source of a mass spectrometer. The matrix is vaporized by a short laser pulse and the analyte molecule is thereby transported into the gas phase in a non-fragmented state. The analyte molecule is ionized by colliding and reacting with the matrix ions generated at the same time. A voltage is applied which accelerates the ions into a field-free flight tube. Due to their different masses, the ions in the ion source are accelerated to different speeds with the smaller ions reaching the detector earlier than the larger ions. The varying times of flight are converted into the different masses of the ions.
An alternative method for ionizing an analyte is electrospray (or ES). Like MALDI, electrospray allows for the ionization/vaporization of polar molecules. Initially the sample of interest is dissolved in a solvent where, to a certain extent, it will exist in an ionized form. In conventional ES, the solution is then pumped through a thin capillary which is raised to a high potential. Small charged droplets are sprayed from the ES capillary into a bath gas at atmospheric pressure and travel down a pressure and potential gradient towards an orifice in the mass-spectrometer high-vacuum system. As the droplets traverse this path, they become desolvated and reduced in size such that surface-Coulombic forces overcome surface-tension forces. As a result, the droplets break up into smaller droplets until either an ion desorbs from the droplet or the solvent is completely removed. The exact mechanism of ion formation is not entirely clear, but the result is a beam of ions, which are sampled by the mass spectrometer. A more detailed description of the ES process is provided in Electrospray Ionization Mass Spectrometry: Fundamentals, Instrumentation and Applications edited by Cole (John Wiley and Sons, New York).
Whichever ionization method used, the formation of unwanted adducts during ionization can compromise the quality and resolution of spectra generated by mass spectrometry. More specifically, the presence of unwanted adducts can make it difficult to detect and analyze an analyte, especially low abundance or low mass analytes.